Predicting pattern critical dimensions in a lithographic exposure process

ABSTRACT

A method for predicting pattern critical dimensions in a lithographic exposure process includes defining relationships between critical dimension, defocus, and dose. The method also includes performing at least one exposure run in creating a pattern on a wafer. The method also includes creating a dose map. The method also includes creating a defocus map. The method also includes predicting pattern critical dimensions based on the relationships, the dose map, and the defocus map.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/313,749, filed Dec. 7, 2011, now U.S. Pat. No. 8,572,518, whichclaims priority to U.S. Provisional Application No. 61/500,520, filedJun. 23, 2011, the contents of both of which are incorporated byreference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to monitoring pattern dimensionscreated by lithographic scanning and, more particularly, to monitoringand recording pattern critical dimensions in situ during the exposureprocess.

BACKGROUND

Photolithography is a process commonly used in defining features duringsemiconductor wafer processing, such as used in the fabrication ofintegrated circuits (ICs). Photolithography generally involves applyinga photoresist material (e.g., resist) to a wafer, exposing the resistusing a pattern of applied radiation (e.g., light), developing theresist, etching a material of the wafer through the patterned resist,and removing the resist after etching. In photolithography, a criticaldimension (CD) is a characteristic length that corresponds to variousfeatures critical to the IC performance that needs to be patterned onthe surface, e.g., a minimum feature width and/or a minimum spacingbetween features. CD control of lithography patterns is an importantaspect of the lithography process to ensure that the end product meetsthe design specification.

Typical methods to record lithography pattern critical dimensions, e.g.,for CD control, are performed ex-situ. Such ex situ methods start withexposing the pattern in the photoresist (e.g., resist), then developingthe resist pattern, and finally measuring the pattern dimensions withthe metrology tool such as a CD scanning electron microscope (SEM) orellipsometer. There are numerous drawbacks with such ex situ methods.For example, there is a significant time delay between pattern exposureand obtaining the actual pattern critical dimensions, since the patternis created using a lithography system and the actual pattern criticaldimensions are later measured using separate metrology tools. Anadditional drawback is that ex situ methods require extra tooling, suchas the metrology tools that generate the pattering performance record.

SUMMARY

In a first aspect of the invention, there is a method of predictingpattern critical dimensions in a lithographic exposure process. Themethod includes defining relationships between critical dimension,defocus, and dose. The method also includes performing at least oneexposure run in creating a pattern on a wafer. The method also includescreating a dose map. The method also includes creating a defocus map.The method also includes predicting pattern critical dimensions based onthe relationships, the dose map, and the defocus map.

In another aspect of the invention, there is a system for predictingpattern critical dimensions in a lithographic exposure process. Thesystem includes a computing device configured to: create a dose map anda defocus map based on data from at least one exposure run that createsa pattern on a wafer; and predict pattern critical dimensions based onthe dose map, the defocus map, and predetermined relationships betweencritical dimension, defocus, and dose.

In another aspect of the invention, a computer program product comprisesprogram code stored in a computer readable medium that, when executed ona computing device, causes the computing device to: create a dose mapand a defocus map based on data from at least one exposure run thatcreates a pattern on a wafer; and predict pattern critical dimensionsbased on the dose map, the defocus map, and predetermined relationshipsbetween critical dimension, defocus, and dose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an illustrative environment for implementing the steps inaccordance with aspects of the invention;

FIGS. 1B and 1C depict relationships between CD, defocus, and dose, inaccordance with aspects of the invention;

FIG. 2 shows an alternative representation of a relationship between CD,defocus, and dose, in accordance with aspects of the invention;

FIG. 3 depicts image formation across a scanner slit captured at apredetermined number of locations in accordance with aspects of theinvention;

FIG. 4 depicts a scanner field exposure composed of patterns exposed inadvancing slit in accordance with aspects of the invention;

FIG. 5 shows a CD map of a pattern in a scanner field in accordance withaspects of the invention;

FIG. 6 shows a CD map arranged in a number of fields across a wafer inaccordance with aspects of the invention;

FIGS. 7A, 7B, and 8 depict a method for constructing sets of Bossungcurves in accordance with aspects of the invention;

FIG. 9 depicts a method for constructing sets of Bossung curves inaccordance with aspects of the invention;

FIGS. 10A-10D depict a method for constructing sets of Bossung curves inaccordance with aspects of the invention;

FIGS. 11A and 12 show block diagrams depicting processes in accordancewith aspects of the invention;

FIG. 11B shows exemplary spacer errors in accordance with aspects of theinvention;

FIG. 13 shows a flowchart of a process in accordance with aspects of theinvention;

FIGS. 14-26 depict details of an illustrative method of forming adefocus map in accordance with aspects of the invention;

FIG. 27 is a schematic view illustrating a photolithography apparatusaccording to aspects of the invention;

FIG. 28 is a flowchart showing semiconductor device fabrication inaccordance with aspects of the invention; and

FIG. 29 is a flow chart showing wafer processing in accordance withaspects of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention relates generally to monitoring pattern dimensionscreated by lithographic scanning and, more particularly, to monitoringand recording pattern critical dimensions in situ during the exposureprocess. According to aspects of the invention, methods and systems areprovided for monitoring and recording the pattern critical dimensionsduring the exposure process, e.g., during operation of the scannerapparatus that creates the patterns. In embodiments, the patterncritical dimensions are determined in situ, e.g., while the wafer isbeing processed in the scanner apparatus, by comparing defocus and dosetraces that are generated by the scanner to predetermined relationshipsbetween defocus, dose, and critical dimension. Implementations of theinvention include constructing sets of Bossung curves that define arelationship between CD, defocus, and dose, constructing defocus anddose maps from data obtained during exposure runs, and predictingpattern CDs based on the Bossung curves and defocus and dose maps. Inthis manner, the scanner patterning performance is determined andrecorded instantaneously during the patterning process, such that thetime to generate the patterning performance record is greatly reducedcompared to ex situ methods. Moreover, implementations of the inventionadvantageously eliminate the need for using separate metrology tools todetermine the scanner patterning performance.

The ability to determine and evaluate the patterning performanceinstantaneously in an IC manufacturing flow, as provided byimplementations of the invention, affords many benefits to themanufacturer. For example, implementations of the invention:

-   -   (i) streamline the IC manufacture in situations where the        process control requires quick determination and judgment of the        patterning performance;    -   (ii) eliminate IC manufacturing delays caused by the need to        wait for metrology tools, e.g., when metrology tools are        occupied or unavailable;    -   (iii) provide information for making a decision whether to        continue with a next step in multi-exposure patterning or repeat        the patterning based on the CD pattern performance in the        preceding steps, e.g., in IC manufacturing processes requiring        double or multiple processes (such as Four-Exposure patterning,        spacer double patterning, Litho-Freeze-Litho-Freeze patterning,        Litho-Etch-Litho-Etch patterning, and similar processes) and in        which each exposure run impacts the outcome of the double or        multiple exposure process;    -   (iv) provide relevant information for processes in which IC        manufacturing involves directed self assembly of patterns and        the pattern exposure on the scanner, preceding the self        assembly, determines the outcome of the self assembled patterns;        and/or    -   (v) provide diagnostics of scanner performance relative to the        imaging process requirements.

As described herein, implementations of the invention provide an in situmethod to predict across wafer CD uniformity (AWCDU), as well as aDouble Patterning (DP) performance monitor and analyzer. In embodiments,scanner traces are used to predict AWCDU. Aspects of the invention maybe used to develop a DP Mapper for predicting the performance of ascanner. In embodiments, DP mapping comprises recording Single Exposure(SE) CD maps with a SE Mapper, and predicting CD maps resulting from asingle exposure of patterns across-wafer. In implementations directed toDP flow, there are two single exposures, and combining two SE maps withtwo exposure overlay data yields a DP map. In implementations directedto multiple exposure flow, such as, for example Quadrupole Exposureflow, there are multiple, single, exposures, and combining multiple SEmaps with multiple exposure overlay data yields a multiple exposure map,such as, for example, quadruple exposure map of CD's.

Exemplary System Environment

The present invention may be embodied as a system, method or computerprogram product and may, for example, take the form of an entirelyhardware embodiment, an entirely software embodiment or an embodimentcombining software and hardware. The present invention may also take theform of a computer program product embodied in any computer-usable orcomputer-readable medium. The present invention can also be implementedas a standalone computer at any site, and may run on a standard personalcomputer, for example.

The computer-usable or computer-readable medium may be, for example, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, device, or propagation medium. Forexample, computer-usable or computer-readable medium may include atangible storage medium, such as, but not limited to: a computerdiskette, a random access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), acompact disc read-only memory (CDROM), and/or an optical storage device.The computer-usable or computer-readable medium may comprise anapparatus that contains, stores, communicates, propagates, or transportsprogram code for use by or in connection with the instruction executionsystem, apparatus, or device. The computer program product may compriseprogram code stored in a computer readable medium that, when executed ona computing device, causes the computing device to perform one or moreof the processes described herein. The computer program product can bewritten in any conventional programming language such as, for example,C++ or the like. Also, the computer program product can be compatiblewith any operating system such as, for example, Windows™ or Linux™operating systems.

FIG. 1A shows an illustrative environment 10 for managing the processesin accordance with the invention. To this extent, the environment 10includes a server 12 that can perform the processes described hereinusing the computer program code. As should be appreciated by those ofskill in the art, the server 12 includes a computing device 14 havingone or more processors 20, memory 22, an I/O interface 24, and a bus 26.The memory 22 can include local memory employed during actual executionof the program code, as one non-limiting example. The server 12 and/orcomputing device 14 can read and/or receive information from a scanner27, and use this information to predict across-wafer CD distribution ofa wafer being processed in the scanner 27. As used herein, the termsscanner and scanner apparatus refer to a photolithography apparatus(e.g., imaging system, exposure apparatus, etc.) used in lithography.

The one or more processors 20 may be dedicated processors programmed forexecution of particular processes or combination of processes inaccordance with the invention, which may be performed on the server 12and/or the computing device 14. The server 12 and/or computing device 14may also be dedicated to particular processes or combination ofprocesses in accordance with the invention. Accordingly, the computingdevice 14 and/or server 12 can include any combination of general and/orspecific purpose hardware (e.g., one or more processors 20) and/orcomputer program code. The server 12 and/or computing device 14 cancommunicate over any type of communications link, such as, for example:wired and/or wireless links; any combination of one or more types ofnetworks (e.g., the Internet, a wide area network, a local area network,a virtual private network, etc.); and/or utilize any combination oftransmission techniques and protocols.

The computing device also includes an I/O device 28 that may be externalto either the computing device 14 or the server 12. The I/O device 28can be, for example, any device that enables an individual to interactwith the computing device 14, e.g., a display (GUI) of a computingdevice. In embodiments, the user can enter information into the systemby way of the GUI (I/O device 28). In one example, the input items canbe accessible to the user by a dialog box. In addition, it iscontemplated that the GUI (I/O device 28) will lead the user through theinput requirements by providing input boxes for textual input or pointeraction.

By way of illustration, the GUI (I/O device 28) can accept scanner andmask information, amongst other information. The scanner information caninclude, for example, user-defined laser wavelength, laser bandwidth,laser spectrum, immersion and dry exposure data, a default index ofrefraction (for water), pupil intensity, immersion exposure, thresholdinformation (e.g., low intensity information from pupilgram files),polarization information, etc. Mask information may include, forexample, editing capabilities for amplitude and phase information, etc.,as well as accepting GDS or OASIS mask files.

The server 12 (and/or computing device 14) includes a centralized devicerepository, e.g., storage system 30. In embodiments, the centralizeddevice repository 30 is configured and/or designed to store the computercode and library information (data) as described below. This allows thesystem and methods to perform the processes described herein.

CD Relationship to Defocus and Dose

Implementations of the invention employ a link (e.g., relationship)between the operation of scanner subsystems controlling focus and doseand critical dimensions of the patterns imaged by the scanner. This linkis through families of CD versus defocus curves for a range of exposuredoses. These curves are referred to as Bossung curves. The informationto accurately construct these relationships in the form of Bossungcurves is obtained by careful calibration of the scanner system setupand careful calibration of the pattern mask. For these conditions, theimaging properties of the scanner slit may be captured by a set of CDversus defocus data generated for a range of exposure doses. Such data,e.g., the family of Bossung curves, allows predictions of the patternCDs by recording image defocus and dose.

FIGS. 1B and 1C present a set of CD versus defocus data, e.g., a familyof Bossung curves, for a particular pattern. In FIG. 1B, the defocusvalue is given at the x-axis, the CD value is given at the y-axis, andeach curve corresponds to a different dose level. Thus, FIG. 1B showspattern CD versus defocus data for a set of doses ranging from Dose1 toDose5. In this manner, FIG. 1B shows a set of Bossung curves uniquelyidentifying an imaging system (e.g., scanner). FIG. 1C is amagnification of a portion of FIG. 1B. The data set thus uniquely linksany pair of defocus and dose (dF/D) with a CD produced by the imagingsystem represented by the graphs.

Families of Bossung curves, such as those shown in FIGS. 1B and 1C, arepattern-specific in the sense that imaging responses of differentpatterns are represented by different shapes of curves. The families ofBossung curves are also particular to the scanner being used in that theline shapes in a particular family of Bossung curves uniquely define thesetup conditions of the imaging system with which they are associated.The shapes of the curves are determined by a wide range ofcharacteristics such as mask type, pattern and CD, illuminator setup andproperties, i.e. illumination layout and polarization, projection lensconditions, i.e. numerical aperture (NA), aberration and apodization,and resist parameters and conditions. Thus, exposure dose and defocusare two drivers of image CD and, for a given imaging setup, the dose anddefocus determine image CD at any location in the image field.

FIGS. 1B and 1C illustrate that for any pair of defocus and dose (dF/D)there is a unique CD value. As the result, the CD values can be uniquelypredicted from dF/D pairs by mapping them on the family of Bossungcurves. Additionally, interpolation in defocus and dose domains may beused to approximate a CD in situations when a dF/D pair is not capturedby the family of Bossung curves.

FIG. 2 shows an alternative way of representing properties of an imagingsystem. In particular, FIG. 2 shows a 2D map of pattern CD versusdefocus and dose constructed from the Bossung curves shown in FIGS. 1Band 1C (FIGS. 1B, 1C and 2 capture the imaging properties of the samesystem). A map, such as that shown in FIG. 2, interpolates CDs for thedefocus and dose combination not included in the domain of the originalBossung curves, e.g., as shown in FIGS. 1B and 1C. In this manner, themap provides a way to determine any CD within the range of defocus anddose of the Bossung curve families. For example, to determine the CDvalues for any combination of defocus and dose, it is sufficient toproject a point (dFi, Di) on the CD surface. The projection of the (dFi,Di) on this surface is a CDi value predicted for the imaging systemscaptured by the family of the Bossung curves from FIGS. 1B and 1C.

The families of Bossung curves may be determined in various ways,including via wafer exposure and metrology, scanner signature, and acombination of scanner signature and wafer exposure and metrology, whichare described in greater detail herein. The wafer exposure and metrologymethod involves performing plural exposures at different focus and doseto create a Focus/Exposure (F/E) array, measuring the CDs (e.g., viametrology) at the locations in the F/E array, and combining this data tocreate the Bossung curves. These Bossung curves are recorded at variouslocations in the scanner image plane. The scanner signature methodinvolves predicting the Bossung curves using imaging simulations basedon known scanner imaging attributes. The scanner signature approachrelies on completeness of the imaging model used for the simulations,e.g., details of the illuminator, mask, and projection lens at variouslocations of the scanner image plane, are predetermined before theimaging model is constructed. Both methods of constructing the familiesof the Bossung curves, e.g., the wafer exposure and metrology method andthe scanner signature method, provide an imaging model representative ofthe imaging conditions at the image location for which the Bossungcurves are generated.

Scanner Imaging

As described above, the imaging properties of a scanner at any locationin the image plane can be represented by family of Bossung curves. Inaccordance with aspects of the invention, this approach is extended torepresent imaging properties of the entire scanner slit and field,which, in turn, is extended to representing image formation across theentire wafer.

Imaging conditions may differ at various locations in the scanner slit.These differences may be due to variations in imaging conditions such asmask pattern and mask CD variation, illumination layout and polarizationvariations, projection lens NA, aberration and apodization variations,scanner flare, and localized resist responses that are induced by theresist process. Implementations of the invention represent imagingproperties of the scanner slit by recording several families of Bossungcurves, e.g., one family of Bossung curves for each location in the slitwhere scanner imaging is to be monitored.

FIG. 3 depicts image formation across a scanner slit captured at apredetermined number of locations in which each location is representedby a different family of Bossung curves. More specifically, FIG. 3 showsa slit diagram in which five locations P1-P5 are identified. FIG. 3 alsoshows a set of five families of Bossung curves B1-B5, each capturing theimaging properties of one respective location P1-P5. As shown in FIG. 3,shapes of CD vs. defocus curves in each of the families B1-B5 differ,reflecting the local differences in the image formation conditionspresent in the scanner slit. Although five slit locations and fivecorresponding families of curves are shown, the invention is not limitedto this number, and any suitable number of locations and families ofcurves may be used within the scope of the invention.

Due to the actual scanner operation and wafer conditions, variouslocations in the slit might be at different defocus. The differences indefocus result from the operation of scanner focus control and levelingsubsystems, the wafer, wafer stage flatness, wafer topography, and imageplane shifts. In addition, various locations in the image slit mightreceive different exposure doses due to illumination non-uniformity. Adefocus and dose pair (dFj/Dj) recorded at each point Pi and referencedto the location family of Bossung curves Bi may be used to uniquelypredict pattern CDj at that location Pi. As such, in accordance withaspects of the invention, a pattern CD map across-slit may be determinedby: constructing families of Bossung curves Bi for each location Pi,recording dFj/Dj pairs representing image formation at each location,and deriving CDj from the Bossung curves and dFj/Dj pairs.

For example, FIG. 3 shows respective families of Bossung curves Biconstructed for locations Pi (P1-P5) of a slit. In embodiments, thefamilies of Bossung curves may be determined as described in greaterdetail herein. FIG. 4 shows dFj/Dj pairs (dF1/D1-dF20/D20) recorded atthe locations Pi. In embodiments, the dFj/Dj pairs are determined fromscanner operation, e.g., from trace data obtained during exposure runs.FIG. 5 shows CDj values (CD1-CD20) corresponding to each respectivedFj/Dj pair using the respective families of Bossung curves constructedfor locations Pi of the slit. In embodiments, each respective CDj valueis derived from the corresponding dFj/Dj pair and family of Bossungcurves Bi.

According to aspects of the invention, FIG. 4 depicts a scanner fieldexposure composed of patterns exposed in advancing slit. Due tooperation of the scanner during full field exposure, the dFj/Dj pairsmay change at each location Pi of the slit. For example, during scanneroperation, the slit advances through the field while the exposureconditions at each point of the slit change. Specifically, the exposuredose and the image defocus may change from one location to another. As aresult, each point in the scanner field may be exposed under differentvalues of defocus and dose, and these respective values are representedin FIG. 4 by the pairs dFj/Dj, and may be referred to as across fieldimaging.

In accordance with additional aspects of the invention, FIG. 5 shows aCD map of a pattern in the scanner field, the CD map comprising the CDvalues CDj (CD1-CD20) and being composed of pattern exposures inadvancing slit. In embodiments, the defocus and dose pairs dFj/Dj fromFIG. 4 are transformed into pattern CDj values using the appropriatefamily of Bossung curves Bi for the location Pi. In implementations, theset of Bossung curves families, such as those in FIG. 3, are used totransform the dose and defocus values delivered by the scannersubsystems into pattern CDs exposed in the scanner field. This isillustrated by FIG. 5 depicting the relationship between the set offamilies of Bossung curves representing imaging conditions of variouslocations in the slit. Accordingly, the set of Bossung curves familiesmay be used to transform dFj/Dj pairs into image critical dimensions CDjfor j=1, 2, 3 . . . . During actual scanner operation, the defocus anddose can differ from one location to another resulting in differentvalues amongst the dFj/Dj pairs, and such variation may result inacross-slit CD distribution via the set of Bossung curves families.

During wafer patterning, the process of described in FIG. 4, e.g.,across field imaging, is repeated at multiple fields across the wafer.For example, while FIG. 4 shows a single field of a wafer, the wafer mayactually comprise a large number of fields Fk (e.g., F1-F79) as shown inFIG. 6. During across wafer imaging, the scanner subsystems delivervarious combinations of patterning defocus and dose (e.g., dFj/Dj pairs)for each respective field Fk. The dFj/Dj pairs for each respective fieldFk may be used to determine an across-wafer distribution of pattern CDsin the same manner as described above with respect to FIG. 5. Theresulting CD map is directly linked to defocus and dose history of thewafer exposure via the set of the Bossung curves families. Inparticular, the across-wafer pattern CD map shown in FIG. 6 is patternedas a sequence of scanner field CD maps.

Each scanner field patterning on the wafer may comprise a sequence ofrepeat events subject to location-specific imaging conditions capturedby the set of Bossung curves families, and continually changing defocusand dose delivered to various locations on the wafer. These continuouschanges of defocus and dose make patterning of each scanner field on thewafer an independent event, recorded in its defocus and dose histories.The image formation conditions at various locations across-wafer maydiffer due to patterning process conditions independent of the scanner,such as across-wafer variations caused by the resist track. Thesedifferences add wafer-process related CD excursions modifying the finalacross-wafer CD map.

Scanner Imaging Representation

FIGS. 7-10D depict methods of determining relationships between CD,defocus, and dose, e.g., families of Bossung curves, in accordance withaspects of the invention. The processes described in this section may beused to characterize imaging conditions across a scanner slit. Inparticular, the processes described in this section may be used togenerate the set of Bossung curves families characterizing imageformation at different locations in the scanner slit. Three methods arepresented for determining the Bossung curves, including: 1) based on thepattern exposure and metrology, 2) involving imaging model imbeddingscanner signatures, and 3) involving imaging model imbedding scanner,and anchored in pattern exposure and metrology.

FIGS. 7A, 7B, and 8 depict the wafer exposure and metrology method forconstructing sets of Bossung curves. This first method relies on a setof actual pattern exposures on the wafer and subsequent patternmetrology, and subsequently fitting Bossung curves to data obtained fromthe pattern exposures and metrology. For example, FIG. 7A shows pluraldata points 200 corresponding to CD values measured using metrologyafter a set of wafer exposures at recorded defocus and dose. Inembodiments, the location of the image in the scanner slit is trackedand the defocus and exposure values are monitored for each pattern.These tests yield the set of data points 200 depicted in FIG. 7A.

After the exposure and metrology data points 200 are collected, a familyof Bossung curves is fit to the data. For example, FIG. 7B shows Bossungcurves 210 fit to the data points 200, e.g., via numerical methods. TheBossung curves 210 represent the imaging conditions at the slit locationat which the wafer exposure and metrology were conducted.

FIGS. 7A and 7B show data points 200 and Bossung curves 210 for a singleslit location. In embodiments, however, data points and a set of Bossungcurves may be determined for more than one slit location. For example,FIG. 8 depicts wafer exposure and metrology data points 200, andcorresponding Bossung curves 210 fit to the data points, for fivedifferent slit locations P1-P5. In particular, FIG. 8 shows exposure andmetrology data points and curve fit results for five different locationsin the scanner slit. The data points 200 at each graph show families ofCD vs. defocus data collected from wafer exposures and metrology. Thelines 210 on the graphs show Bossung curves fitted to the wafer exposureand metrology results. The set of families of Bossung curves representsthe imaging conditions at the five positions P1-P5 across the slit.Though very effective, the wafer exposure and metrology methodology hasa disadvantage of requiring a high amount of expensive exposure andmetrology resources, in terms of both equipment and manpower. Thisdisadvantage is compounded by the fact that the wafer exposure andmetrology data is generated anew for each pattern of interest.

FIG. 9 depicts a second method for constructing sets of Bossung curvesin accordance with aspects of the invention, i.e., the scanner signaturemethod. In the scanner signature method, the Bossung curves aredetermined through modeling of the scanner using predetermined imagingattributes of the scanner. For example, when the imaging attributes fora scanner are completely characterized, i.e., all of the scannersubsystems contributing to image formation are tested and quantified,the family of Bossung curves may be predicted by an imaging modelimbedding the scanner attributes, e.g., scanner signatures. To constructthe set of families of Bossung curves capturing imaging properties ofmultiple locations in the slit, the scanner attributes specific to theselocations are recorded. When this is done, the scanner slit imaging canbe represented by the data set such as the one shown in FIG. 9.

In particular, FIG. 9 shows sets of Bossung curves 220 that arepredicted by a scanner imaging model using attributes of the scanner atdifferent slit locations P1-P5. Once the imaging attributes of thescanner are recorded, this method of characterizing the imaging in thescanner slit is very efficient, and can be applied for a wide range ofpatterns. To properly capture the impact of wafer process on theimaging, the model setup requires some metrology anchor. The anchor isaccomplished by exposures and metrology tests for particular setuppatterns, and fitting some of the model adjustable parameters to bestmatch the modeling results with the corresponding metrology results.

A third methodology for constructing a set of Bossung curves isillustrated in FIGS. 10A-10D and involves a combination of the waferexposure and metrology method and the scanner signature method. Thisthird method combines the efficiency of scanner signature method with alimited anchor done specifically for the pattern of interest. Thispattern specificity differentiates it from general scanner signaturebased method, and captures the imaging response of the pattern ofinterest. In embodiments, this third method of constructing the set offamilies of Bossung curves representing imaging properties of multiplelocations in the slit comprises four steps as depicted in FIGS. 10A-10D.

Step 1 is depicted in FIG. 10A and comprises wafer exposure, metrology,and curve fitting for one location of the scanner slit, e.g., in themanner described above in FIGS. 7A and 7B. In embodiments, Step 1involves imaging the patterns of interest in one location of the slit,e.g., the slit center (P3). The exposures are performed to capture apattern CD as a function of defocus at multiple exposure (dose) levels.The patterns are processed and CD metrology is conducted. The resultingfocus exposure data points 200 are fitted to a family of Bossung curves210. This family is imaging reference representing slit center.

Step 2 is depicted in FIG. 10B and comprises generating scannersignature based slit response. In embodiments, Step 2 involves settingup imaging models imbedding scanner signatures for multiple locations inthe slit, including the reference location where the pattern exposureand metrology in Step 1 were performed. These models are then used tosimulate sets of Bossung curves 220 at the other slit locations P1, P2,P4, and P5, each capturing the imaging properties of the location forwhich the scanner signature is imbedded in one of the models.

In accordance with aspects of the invention, the simulated Bossungcurves 220 for the reference location P3 are linked with the Bossungcurves 210 constructed in Step 1. This may be accomplished by aligningthe model dose values with metrology values in such a way that the twoBossung families are aligned. Once the dose scales are aligned, theremaining families of location-specific Bossung curves are calculatedfor the dose values set.

Step 3 is depicted in FIG. 10C and comprises extracting across-slitresponse excursions 230. In embodiments, Step 3 comprises calculating aset of location-specific excursions of the Bossung curves. Theseexcursions are in dCD vs. defocus format for the dose set established inSteps 1 and 2, where dCD is the difference between thelocation-specific, simulated CD and a corresponding modeled CD in thereference location, e.g. the scanner slit center P3

Step 4 is depicted in FIG. 10D and comprises combining metrology andslit response excursions. In embodiments, Step 4 comprises calculatingthe set of families of Bossung curves. For the locations other than thereference location, e.g., slit center, this is done by addinglocation-specific dCD's calculated in Step 3 to the family of referenceBossung curves fitted to the data collected in Step 1. These sums oflocation-specific excursions and the reference Bossung familiesrepresent imaging responses of various locations in the slit. It issupplemented by Bossung family fitted in Step 1 forming a complete setof Bossung curves families for the slit, combining the metrology anchorand scanner signature-based imaging models predictions.

According to other aspects of the invention, an alternative to Bossungcurve method is interpolation among the metrology data points. In thismethod, the fit of Bossung curves to the metrology data is notperformed. Instead, in embodiments, the imaging response of a scanner isrecorder as a discreet set of CD responses to a discrete set of dose anddefocus pairs. This approach allows one to find the CD at any given doseand defocus pair by interpolating the CD among the imaging responses atthe discrete dose and defocus pairs. This interpolation method appliesto metrology and scanner signature methods of constructing the scannerimaging model in accordance with aspects of the invention.

Scanner Variations and Drifts

This section describes changes in the imaging properties of the scannercaused by instantaneous fluctuations and evolutionary drifts of thescanner subsystems. Techniques for accounting for these variations inaccordance with aspects of the invention are also disclosed.

Under normal operating conditions, the set of the Bossung curvesfamilies representing imaging condition at various points in the slit issubstantially constant. However, two aspects of scanner operations maycause changes in the imaging properties of the scanner. These aspectsare referred to instantaneous variations and evolutionary drifts.

Instantaneous variations are minute, instantaneous variations of thescanner conditions caused by the dynamics of the wafer exposure. Scannerdynamic operation results in a range of fast fluctuations around thenominal imaging conditions. These fluctuations occur in level ofexposure, scan synchronization, image locations and are compounded byimage plane tilt and resist effects such as diffusion of the moleculesdriving the photochemistry of resist exposure. All of these impacts leadto instantaneous and minute changes in the image contrast and, thereforeresult in CD excursions. The scale of these excursions is small ascompared to primary components of imaging system, i.e., imaging dose anddefocus.

This range of engineering, physic and chemistry phenomena is difficultto account for on the basis of first principle. Instead,phenomenological treatment is more effective. Some of these excursionscan be represented as instantaneous dose and defocus variations. Assuch, in embodiments of the invention, these instantaneous variationsare accounted for by convolving the CD estimates for the imaging systemsat the defocus and dose values nominal for a given imaging event, withthe defocus and dose distributions representing dynamics of the exposureprocess. In embodiments, this image correction is represented by formula1:CDi _(dyn)(dFi,Di)=∫∫CDi _(nom)(dFi−f,Di−e)p(f)p(e)dfde   (1)

In formula 1, CDi_(dyn)(dFi, Di) is CD corrected for dynamic operationof the scanner pattering at the nominal dFi/Di conditions.CDi_(nom)(dFi,Di) is the nominal CD, i.e. the value of image CD notimpacted by the dynamics of scanner operation, p(f) and p(e) are thedistributions of defocus and dose around the nominal conditions. p(e) isestimated from trace of the dose irradiance delivered to the wafer. p(f)is estimated from the focus monitor traces, image plane tilt in the scandirection, and diffusion coefficients of the resist molecules. Inembodiments, the double integration in formula 1 is performed over theranges of defocus and exposure dose variations representative of thescanner operations, i.e. the ranges determined by the scanner operation.

Evolutionary drifts are changes of the scanner subsystems during theiroperational lifetime. During the scanner life, its imaging propertiescontinually change, albeit such changes are typically very slow and veryminute. In particular, changes in one or more of the illuminator,projection lens, and scanner alignment may lead to changes in theimaging properties of the scanner. These changes manifest themselves,for example, as changes in illuminator pupil fill and its uniformityacross slit, changes in effective NA of the projection lens, changes inprojection lens aberration residue, changes in pupil telecentricity,etc. In embodiments of the invention, the impact of imaging propertieschanges on CD maps is captured through the set of Bossung curvesfamilies representing imaging properties of the scanner. Inimplementations of the invention, these changes are accounted for byperiodically repeating the processes for constructing the set of Bossungcurves families, e.g., as described with respect to FIGS. 7-10D.

Scanner Defocus and Dose Control

This section describes various contributors to defocus and dose controlduring scanner operation, including methods for constructs the imagedefocus and dose maps across scanner slit, across-field, andacross-wafer in accordance with aspects of the invention. Variousscanner functions determining distribution of defocus and dosevariations during the image formation are described, as links betweenvarious scanner functions, defocus, and dose accuracies. This sectionalso describes which aspects of dose and defocus control result insystematic or statistical contributions.

Furthermore, this section describes: (1) signals generated by thescanner subsystems that carry the information on the exposure defocus;(2) aspects of scanner operation that impact the accuracy of focuscontrol; (3) signals generated by the scanner subsystems that carry theinformation on the exposure dose; and (4) aspects of scanner operationthat impact the accuracy of dose control. When predicting a CD map inaccordance with aspects of the invention, items (1) and (3) are used toidentify direct inputs to predict the CDs, and items (2) and (4) lead toestimates of the CD prediction accuracy resulting from the operation ofvarious scanner subsystems.

The data and impacts are described in this section as either static ordynamic. As used herein, static means the data and/or impact does notvary during the exposure across scanner field and across-wafer, whiledynamic means the data and/or impact can vary during the exposure. Inembodiments, the static data is recorded prior to the actual exposure,and the dynamic traces are monitored continuously during the exposure.This section, however, does not describe two other functions enhancingfocus control performance: Iterative Learning Correction (ILC) andScanner Enhanced Focus Function (SEFF). ILC is not addressed because itenhances scanner ability to minimize the image defocus, and as such, itdoes not provide the actual values image defocus or defocus accuracy.SEFF is not addressed because it enhances the performance of Auto Focus(AF) system but it does not provide information on the actual AFperformance.

Three issues underlying imaging defocus control include: location of theimage plane determined by the projection lens; location of the waferrelative to the physical location of the projection lens; and variouscorrections (e.g., DIMG—setup offsets to match a target performance andDCRS—reticle bow during scan, AMEN—corrections dependent on thedirection of the scan) applied to wafer z-position to correct forreticle z-movement. These three defocus corrections are independent ofeach other and are tracked separately.

The determination of the lens image plane is made through the aerialimage sensor (AIS) and Wafer Table Direct Interferometer (WTD). The AISsenses the location of the lens focus while WTD calibrates the distanceof that focus from the physical location of the lens. Together, theyprovide the location and the scale of z-position of the image planerelative to the physical location of the projection lens. This data isstatic. WTD setup affects focus and leveling performance across thewafer. Poor or improper WTD setup results in focus and tilt errors,i.e., WTD error contributes to dynamic focus errors. In embodiments, theWTD interferometer information is monitored during exposure.

The distribution of image plane position across the slit is representedby the field curvature recorded during Contrast Focus (CF), AIS tests,or PSFM tests. This data measures the focus position results for thefocal plane variations across scanner slit. This data is a static.

Auto Focus (AF) traces are collected for a number of locations (e.g.,five locations) across slit just prior to the actual exposure. These AFtraces are used to form z-position targets for the wafer during theexposure by the wafer z-position control system. The actual z-positionof the wafer, relative to the physical position of the lens, ismonitored during the exposure by WTD continuously probing how close theposition targets are met by the wafer z-position system. In this sense,these WTD traces are dynamic.

The AF sensor offsets are calibrated by ASOC sensor. This data containsthe information on offsets of the wafer positions targets relative tothe AF sensor positions. This information is specific to the location inthe scanner slit. The ASOC data is static.

Another impact to the defocus is due to DIMG and DCRS functions forcorrecting reticle z-displacement during the scan. These functionscorrect for the reticle z-movement by adjusting wafer z-position duringthe scan. DIMG drives the wafer position correction based on the priorexposure data. Reticle DCRS drives DIMG corrections specific to thereticle. AMEN wafer corrections are based on the direction of the scan.In embodiments, DIMG, AMEN and Linear Tilt errors are evaluated andcorrected if needed. Sometimes the DIMG and AMEN errors are so smallthat no corrections are actually applied. DCRS can correct for the Zshape of specific reticles. These errors are typically not very large,but if ignored could contribute focus error. If any of these correctionsare not applied, they need not be accounted for during image defocusanalysis.

The composite of image plane distribution across—slit and the WTDtraces, and when applicable, the image plane corrections provide thebasis for nominal defocus maps across-slit, across-field andacross-wafer. This composite is dynamic.

The position of the wafer relative to lens image plane during exposureof the images in the scanner is monitored by WTD sensor (wafer tabledirect interferometer). During the pattern exposure a scanner may besubject to defocus variations during exposure. Fluctuations of the WTDtraces showing the wafer position relative to the image plane along thescan direction, provide a basis for defocus variation distribution andrange estimates across-slit, across-field and across-wafer. Thiscomposite is dynamic. In embodiments, this defocus distribution may bemodified to account for the dynamics of the resist moleculesparticipating in exposure process photochemistry. These resist relatedfactors determining p(f), the defocus distribution around the nominal,described above in formula (1).

Defocus accuracy may be affected by a number of scanner functions. Forexample, ASOC calibration accuracy determines how accurately the AF setsthe wafer z-position targets, which is a static impact.

Aerial Image Sensor (AIS) accuracy and repeatability determines howaccurate AF signals were calibrated to identify the lens image plane.This accuracy is impacted by the AIS fiducially degradation. This is astatic impact.

Accuracy and repeatability of AIS calibration determines accuracy oflens best focus measurement. This is a static impact.

Infrared Aberration Control (IAC) determines how accurately lens focusis corrected to account for dipole illuminator induced astigmatism. Thisis a dynamic impact.

Lens Heat Controller (LHC) determines how accurately the image plane iscorrected for the lens thermal loads. This is a dynamic impact.

WTD accuracy of calibrating the z-translation of the wafer may be astatic or a dynamic impact.

Scan Synchronization z-MSD determines how accurately the waferz-position is held during the exposure. This is a dynamic impact.

Two issues underlying dose control include across slit dose distributionand across field dose control. Both of these issue impact dose deliveryin the scanner field and across wafer.

Illumination uniformity across slit traces distribution of dose atvarious locations in the slit. Illumination uniformity data is availablefrom a slit uniformity test. This is a static impact. When dosefluctuates during the scanner operation, e.g. during field exposuresacross wafer, the illumination uniformity does not change.

Dose delivered to the reticle is monitored by the dose integrator. Thedose integrator output signal is a direct trace of the exposure dose.This is a dynamic impact.

Nominal dose distribution across field and image is a composite ofuniformity trace along the slit and integrator trace along the scandirection. This is a dynamic impact.

A scanner may also experience dose fluctuations not captured by the doseintegrator during exposure. In embodiments, the exposure irradianceintegrator trace along the scan direction provides the basis for dosevariation distribution and range estimates across-slit, across-field andacross-wafer. This is a dynamic impact. These factors determine p(e),the dose distribution around the nominal, in formula (1).

Dose accuracy is impacted by a number of scanner characteristics. Forexample, dose accuracy may be impacted by the accuracy of integratorcalibration with Molectron wafer. This is a static impact.

The dose controller accuracy may also be dependent on scan speed, NDfilters, pulse energy stability, slit width etc. This is a dynamicimpact.

The dose accuracy may also be impacted by the accuracy of a NewPredictive Dose Controller, correcting transient effects in the lenstransmission. This is a transient therefore a dynamic impact.

Block and Flow Diagrams

FIGS. 11A and 12 depict block diagrams illustrating processes forconstructing a Single Exposure (SE) CD Map and a Double Patterning (DP)Space Map and Line Map in accordance with aspects of the invention. Thesteps of FIGS. 11A and 12 may be implemented in the environment of FIG.1, and may be performed using techniques described with respect to FIGS.2-10.

Referring now to FIG. 11A, step 1105 comprises characterizing theimaging response of the scanner. In embodiments, step 1105 includesconstructing static field Bossung curves 1107, e.g., as described abovewith respect to FIGS. 7-10.

Step 1110 comprises conducting test runs of the scanner to gather datato determine characteristics of the scanner. In embodiments, this dataincludes: illumination uniformity data 1112 representing doesdistribution across the slit; total field deviation (TFD) data 1113detected by AIS and calibrated by WTD; and focus correction data 1114,including DIMG, DCRS, AMEN, and Linear Tilt correction data.

In embodiments, steps 1105 and 1110 are performed prior to themanufacturing exposure runs for a particular wafer. The data obtained insteps 1105 and 1110 may be stored, e.g., in storage system 30 as shownin FIG. 1, and later accessed for use during in situ determination of CDvalues for a wafer undergoing manufacturing exposure runs.

Step 1116 comprises accessing and calibrating scanner traces. Inembodiments, step 1116 includes a Dose Integrator trace calibration andWTD trace calibration, which may be performed in conventional manners.Step 1117 comprises linking locations across the wafer with locations inthe Focus/Dose traces.

Step 1120 comprises performing the exposure runs (e.g., themanufacturing lithographic exposures) on the wafer to expose a patternin a resist on the wafer. Performing the exposure runs at step 1120includes collecting the scanner traces, such as the Auto Focus and WTDtrace 1122 used for scanner defocus monitoring and the Dose Integratortrace 1123 used for monitoring dose delivered to the reticle. Performingthe exposure runs at step 1120 may also include collecting ScanSynchronization data 1124, such as MSDx, MSDy, and MSDz, which maycontribute to image blur.

At step 1129, a dose map 1130 is created from the scanner traces, inparticular the Dose Integrator trace data 1123. In embodiments, the dosemap is a data structure defining respective doses applied at respectivelocations across the wafer during the exposure runs.

At step 1134, a defocus map 1135 is created from the scanner traces andtest data, in particular the Auto Focus and WTD trace data 1122, TFDdata 1113, and focus correction data 1114. Creating the defocus map isdescribed in greater detail herein with respect to FIGS. 14-26.

At step 1140, an SE CD map 1145 is created using the dose map, defocusmap, and Bossung curves 1107. In embodiments, the SE CD map 1145comprises a data structure that defines a plurality of predicted CDvalues at locations across the wafer. The predicted CD values may bedetermined in the manner described above with respect to FIGS. 3-6. Forexample, the dose map 1130 and defocus map 1135 may be used to define atleast one array 1150 of defocus and dose pairs, e.g., dF/D,corresponding to locations on the wafer and positions of the scannerslit. Each dF/D pair in the array 1150 may be used in conjunction withthe appropriate Bossung curves 1107 to determine a CD value for alocation on the wafer, resulting in an array of determined CD values inwhich each CD value is linked to a location on the wafer, e.g., the SECD map 1145.

In embodiments, the SE CD map 1145 may be corrected to account for knownresist track impacts resulting in CD modifications across wafer. Such acorrected map would represent the combined impact of scanner and resisttrack on AWCDU, and may be used in implementations of the invention whendetermining CD values.

Step 1140 may optionally include step 1140a, which comprises correctingthe predicted CD values for the impact of defocus and dose blur. Step1140 a may include, for example, applying a blur function 1155 whendetermining the CD values, the blur function 1155 being based on atleast one of Dose Integrator data 1123 and Scan Synchronization data1124.

In accordance with aspects of the invention, the SE CD Map 1145 is anAWCDU map and can be used in single exposure (SE) processes to extractSE pattern statistics, e.g., at step 1157. The SE CD Map 1145 andpattern statistics may be used for at least one of scanner monitoring,tuning, and troubleshooting.

In implementations utilizing double patterning (DP), step 1160 comprisesusing the SE CD Map 1145 and a Spacer Map 1165 to construct a DP SpaceMap 1170 and DP Line Map 1175. In embodiments, the Spacer Map 1165comprises a data structure that defines how plural exposures aresuperimposed on the wafer. Step 1160 may comprise performing a CD erroranalysis of spacer DP errors, such as, for example, pattern gaps 1180,pattern lines 1181, and lines overlay 1182, which are shown in greaterdetail in FIG. 11B. In embodiments, the DP Space Map 1170 and DP LineMap 1175 comprise data structures defining line and space CD predictionsfor the pattern exposed in step 1120.

At step 1185, DP space and line statistics are extracted from the DPSpace Map 1170 and DP Line Map 1175. As with the SE statistics, the DPstatistics may be used for scanner performance predictions (e.g.,predicting the scanner contribution to space CD and spacer lineoverlay), scanner performance troubleshooting (e.g., decoupling scannerand spacer process contributions, scanner analysis across-slit,across-field, across-wafer, etc.), and scanner performance monitoring(e.g., dose and focus trace analysis is the basis for judgment of thescanner stability and condition).

FIG. 12 depicts a block diagram illustrating another process forconstructing SE CD Maps and a DP Space Map and Line Map in accordancewith aspects of the invention. FIG. 12 includes steps having the samereference numerals as steps shown in FIG. 11A, and these steps areperformed in the same manner. As depicted in FIG. 12, the SE mappingstep, e.g., step 1140, results in two SE CD maps, e.g., SE DC Map 1(1145 a) and SE DC Map 1 (1145 b). The two SE CD maps 1145 a and 1145 bcomprise predicted CD values, e.g., determined as described herein, forseparate single exposure patterning processes.

Still referring to FIG. 12, the DP mapping step, e.g., step 1160′,comprises deriving the DP Space Map 1170 and DP Line Map 1175 from theSE CD maps 1145 a , 1145 b and an Overlay Map 1190. The DP mapping step1160′ in FIG. 12 is performed similar to the DP mapping step 1160 ofFIG. 11A, but uses different inputs. In embodiments, the Overlay Map1190 comprises scanner data defining how the two single exposureprocesses are superimposed. The Overlay Map 1190 represents raw datafrom which a Spacer Map, e.g., Spacer Map 1165 of FIG. 11A, may beconstructed.

According to aspects of the invention, the dose map 1130 and defocus map1135 are determined in situ (e.g., during lithographic processing in thescanner) of a particular wafer. In embodiments, the dose map 1130,defocus map 1135, and Bossung curves 1107 are used to predict CD valuesat locations on the wafer for constructing the SE CD Map 1145, DP SpaceMap 1170, and DP Line Map 1175, which is also performed in situ. The CDvalues of the constructed maps can be compared to the CD values in thedesign specification to determine whether the patterned wafer meets thespecification. In this manner, implementations of the invention can beused to predict pattern CD values across the entire wafer while thewafer is undergoing the lithographic exposure runs (e.g., in situ) andwithout resorting to post-exposure metrology (e.g., ex situ).

FIG. 13 depicts a flow diagram illustrating a process in accordance withaspects of the invention. Step 1305 comprises scanner model setup andcalibration. In embodiments, step 1305 includes characterizing theimaging response of the system by constructing static field Bossungcurves, e.g., as described at step 1105. Step 1305 may additionallycomprise accessing and calibrating scanner traces, e.g., as described atstep 1116, and linking locations across the wafer with locations in thefocus/dose traces, e.g., as described at step 1117.

At step 1310, the lithographic exposures are run and traces arecollected, e.g., as described above at step 1120. At step 1320, thedefocus and dose maps are constructed from the traces and test data,e.g., as described above with respect to steps 1129 and 1134. At step1330, the defocus and dose maps are transformed into predicted CD data,e.g., an SE CD Map, using the Bossung curves, e.g., the Bossung curvesderived at step 1305 which represent the imaging properties of thescanner for this pattern. Step 1330 may be performed in a manner similarto step 1140, described above, and may optionally include correcting thepredicted CD data for blur, e.g., as described at step 1140 a. At step1340, the single exposure pattern statistics are extracted from the SECD map, e.g., as described with respect to step 1157. The singleexposure pattern statistics may be used for at least one of: in situmonitoring the SE patterning process (e.g., comparing predicted patternCD values to specification CD values), in situ scanner troubleshooting,and in situ scanner tuning.

At step 1350, the SE CD Map and a Spacer Map are used as input forconstructing a DP Spacer Map and DP Line Map, e.g., in a manner similarto that described with respect to step 1160 in FIG. 11A. Alternatively,step 1350 may comprise using two SE CD Maps and an Overlay Map toconstruct the DP Spacer Map and DP Line Map, e.g., in a manner similarto that described with respect to step 1160′ in FIG. 12. At step 1360,the DP statistics are extracted from the DP Spacer Map and DP Line Map,e.g., as described with respect to step 1185. As with the singleexposure pattern statistics, the DP statistics may be used for at leastone of: in situ monitoring of the DP process (e.g., comparing predictedpattern CD values to specification CD values), in situ scannertroubleshooting, and in situ scanner tuning.

Constructing Dose map and Defocus Map

This section describes data and scanner traces used to construct thedose maps and defocus maps in accordance with aspects of the invention.The trace maps in turn, may be used to create the SE maps representingacross wafer maps and distributions, as described herein.

In embodiments, dose maps are created using illumination uniformitydata, dose integrator data, and predictive dose controller data. Theillumination uniformity data may comprise, for example, datarepresenting dose distribution across the slit. The dose integrator datamay comprise, for example, data obtained by monitoring dose delivered tothe reticle including Predictive Dose Controller data. In embodiments,the predictive dose controller data may comprise, for example, datacorrecting transient effects in the lens transmission, data defining howthe transient correction is determined and applied to modify theexposure dose target.

According to aspects of the invention, defocus maps are created using atleast one of: WTD data, TFD data, focus correction data, autofocus (AF)trace data, AF offset data, reticle sag data and scanner test data. Inembodiments, the WTD data is used for scan monitoring and TFDcalibration. The WTD data may comprise data defining at least one of:how the WTD accuracy and repeatability are tested; the number andcoordinates of points at which WTD traces are recorded; how to map theWTD traces to various points across scanner slit; how the WTD data isreferenced to the physical lens position; what the zero of the data setrepresents, e.g., a physical reference point in the scanner or the lenspupil; how the trace signal scales to the physical distance inz-direction of the scanner; and which points in the trace represents thefield leading edge and closing edge.

In embodiments the TFD data is detected by AIS calibrated to WTD. TheTFD data may comprise data defining at least one of: a definition bywhich TFD is measured; how the data is referenced to the physical lensposition; what the zero of the data set represents, e.g., a physicalreference point in the scanner or the lens pupil; other metricsrepresenting distribution of lens focal plane across the scanner slit.

In embodiments the focus correction data comprises DIMG, DCRS, AMEN, andLinear Tilt data. The focus correction data may further comprise datadefining at least one of the following for each of DIMG, DCRS, AMEN, andLinear Tilt data: how the DIMG, DCRS, AMEN, and Linear Tilt datacorrections are applied, e.g., as a straight offset to the waferposition or by some other formula; whether the data represents a set ofwafer plane adjustments or a data set specifying adjustments atpreselected slit points; how these data are indexed to the AF sensors,e.g., whether the corrections are made at the sample points defined bythe AF or at some other correction plan; and how the data relates to thephysical scale in z-direction of the scanner.

In embodiments the AF trace data comprises data defining AF tracescollected at a predetermined number (e.g., five) of locations across thescanner slit just prior to the actual exposure. The AF trace data maycomprise, for example, data defining at least one of: which points inthe trace represent the field leading edge and closing edge; and how thetrace signal scales to the physical distance in z-direction of thescanner.

In embodiments the AF offset data is calibrated by ASOC. The AF offsetdata may comprise, for example, data defining at least one of: a set ofAF calibration offsets or other calibration traces; and how these dataare indexed to the AF sensors.

In embodiments the scanner test data comprises data defining at leastone of: ASOC, AIS, IAC, LCH, and scan synchronization data (e.g., MSDx,MSDy, and MSDz). The scanner test data may comprise data definingmetrics for evaluating accuracy and repeatability of each of the ASOC,AIS, IAC, LCH, and scan synchronization data.

Image Defocus Budget and Map

FIGS. 14-26 depict details of an illustrative method of forming adefocus map in accordance with aspects of the invention. In embodiments,image defocus is the distance from a shot surface to a field curvature,and may be determined as a vector sum of: (i) topography target errors,(ii) stage trajectory errors, and (iii) image plane excursions from theprojection optic (PO) focus plane. These aspects of image defocus aredescribed in detail with respect to FIGS. 14-26.

As shown in FIGS. 14 and 15, a wafer surface is not flat. For example,FIG. 14 shows a plan view of a topography of a wafer 1410 in whichdifferent shading defines different heights of the wafer upper surface.The wafer topography may be calculated from traces recorded by anautofocus (AF) sensor of the scanner. FIG. 14 shows the wafer 1410divided into a number of fields 1415. The dotted line 1420 represents asingle trace path, and the circle 1425 identifies a particular shotcross section along the path.

FIG. 15 shows a wafer cross section 1510 of the wafer 1410 atop a waferstage 1515 of the scanner. The wafer cross section 1510 corresponds to across section of the wafer along the line 1420 from the top view of FIG.14, a portion of which is shown for illustration in FIG. 15. Shown abovethe wafer 1410 a projection optic (PO) 1520 that performs the AF scan.The PO 1520 optically detects a shot surface 1525, which characterizesthe topography of a portion of the upper surface of the wafer at theinstantaneous exposure position of the PO 1520. Shown enlarged and tothe right of the wafer cross section 1510 is a diagrammatic relationshipbetween the shot surface 1525 and a focus target 1530 of the PO 1520. Inembodiments, the scanner determines the focus target 1530 as a singlestraight line based on the spatial orientation of the shot surface 1525.

As depicted in FIG. 16, the scanner determines the focus target 1530 byminimizing the distance between the shot surface 1525 and the PO focusplane. For example, vectors 1610 represent deviations between the focustarget 1530 and the shot surface 1525, and are referred to as shotsurface excursions 1610. Starting with the shot surface 1525, thescanner determines a focus target 1530 by identifying a straight linethat minimizes the sum of the absolute values of the shot surfaceexcursions. In particular embodiments, and as depicted in FIG. 17, thescanner determines the respective focus target 1530 by fitting a firstorder line to the non-flat shot surface 1525. Although a single shotsurface and focus target are shown, it is understood that the scannerperforms this function for each respective shot surface along each traceof the wafer.

FIG. 18 depicts the auto focus operation. In embodiments, the scanneradjusts the tilt and offset of the wafer stage 1515 to substantiallyalign the focus target 1530 with a focus plane 1810 of the PO 1520.

FIG. 19 shows topography target errors 1910 in accordance with aspectsof the invention. As described above, the scanner autofocus operationaims to align the AF target plane 1530 with the PO focus plane 1810. Theorientation and position of the AF target plane 1530 determines thewafer stage tilt and offset to substantially align the shot surfacerelative to the PO. The scanner calculates the topography target errorsafter performing the wafer stage tilt and offset. In embodiments, thetopography target errors are determined as the difference between the AFtarget plane 1530 and the shot surface 1525.

FIG. 20 shows a plan view of a graph 2010 of topography target errorsfrom an entire wafer, in which different shades depict different amountsof topography target error at particular shot surfaces. In embodiments,the topography target errors are wafer specific, are impacted by theparticular process, and are recorded by the AF sensor prior to waferexposure.

FIG. 21 depicts stage trajectory errors 2105 in accordance with aspectsof the invention. As described above, the scanner autofocus operationaims to align the AF target plane with the PO focus plane 1810. However,exact alignment of the AF target plane with the PO focus plane 1810 israrely achieved, and in actuality the AF target plane typically lies ina plane 2110 that is slightly misaligned to the PO focus plane 1810.Deviations between the actual AF target plane 2110 with the PO focusplane 1810 are referred to as stage trajectory errors 2105. Inembodiments, the stage trajectory errors are recorded by the “Z”position sensors of the scanner. FIG. 22 shows a plan view of a graph2210 of stage trajectory errors for a wafer. In embodiments, the stagetrajectory errors are determined by the stage operations as monitored bythe WTD.

Image plane excursions are a third component of the overall imagedefocus. Image plane excursions arise from image curvature of the PO1520. As depicted in FIG. 23, image curvature 2310 includes twocomponents: Petzval curvature 2315 and image bending 2320. Petzvalcurvature 2315 is an optical aberration that remains constant along thescan, and which can be calculated or measured using known techniques.Image bending 2320 is caused by reticle sag and may be variable alongthe scan. FIG. 23 shows that the image curvature 2310 is a vector sum ofthe Petzval curvature 2315 and image bending 2320 deviations from the POfocus plane 1810 along the PO focus plane 1810. The image curvature 2310results in the image plane where images are formed being different fromthe PO focus plane. The discrete differences are quantified as imageplane excursions 2410 depicted in FIG. 24. In embodiments, the imageplane excursion is specific to the imaging conditions, impacted by theimaging setup, and composed of Petzval curvature and reticle imagebending contributions.

According to aspects of the invention, the image defocus 2505 is thedistance from the wafer surface 1525 to the field curvature 2510, asshown in FIG. 25. In embodiments, the image defocus 2505 is a vector sumof topography target errors 1910, stage trajectory errors 2105, andimage plane excursions 1810 from the projection optic (PO) focus plane1810, as depicted in FIG. 26. In particular embodiments, the sum oftopography target errors and the stage trajectory errors is relative toscanner zero-defocus. Additionally, the image plane excursions from thePO focus plane are calibrated relative to scanner zero-defocus.

According to aspects of the invention, using the methodology describedwith respect to FIGS. 14-26, a value of image defocus may be determinedfor any location (e.g., shot surface) on the wafer. By determining aplurality of image defocus values at a plurality of locations, and bylinking the each respective image defocus value with a correspondinglocation, an image defocus map (e.g., defocus map 1135) may beconstructed.

Exemplary Photolithographic Apparatus Implementing Aspects of theInvention

FIG. 27 is a schematic view illustrating a photolithography apparatus 40(e.g., scanner, imaging system, exposure apparatus, etc.) in accordancewith the present invention. The wafer positioning stage 52 includes awafer stage 51, a base 1, a following stage and following stage base 3A,and an additional actuator 6. The wafer stage 51 comprises a wafer chuckthat holds a wafer W and an interferometer mirror IM. The exposureapparatus can also include an encoder to measure stage position. Thebase 1 is supported by a plurality of isolators 54 (or a reactionframe). The isolators 54 may include a gimbal air bearing. The followingstage base 3A is supported by a wafer stage frame (reaction frame) 66.The additional actuator 6 is supported on the ground G through areaction frame. The wafer positioning stage 52 is structured so that itcan move the wafer stage 51 in multiple (e.g., three to six) degrees offreedom under precision control by a drive control unit and systemcontroller, and position and orient the wafer W as desired relative tothe projection optics 46. In this embodiment, the wafer stage 51 has sixdegrees of freedom by utilizing the Z direction forces generated by thex motor and the y motor of the wafer positioning stage 52 to control aleveling of the wafer W. However, a wafer table having three degrees offreedom (Z, θx, θy) or six degrees of freedom can be attached to thewafer stage 51 to control the leveling of the wafer. The wafer tableincludes the wafer chuck, at least three voice coil motors (not shown),and bearing system. The wafer table is levitated in the vertical planeby the voice coil motors and supported on the wafer stage 51 by thebearing system so that the wafer table can move relative to the waferstage 51.

The reaction force generated by the wafer stage 51 motion in the Xdirection can be canceled by motion of the base 1 and the additionalactuator 6. Further, the reaction force generated by the wafer stagemotion in the Y direction can be canceled by the motion of the followingstage base 3A.

An illumination system 42 is supported by a frame 72. The illuminationsystem 42 projects radiant energy (e.g., light) through a mask patternon a reticle R that is supported by and scanned using a reticle stage.Alternatively, in the case of systems using extreme ultraviolet (EUV)radiation, radiant energy is reflected by the reticle R. The reticlestage may have a reticle coarse stage for coarse motion and a reticlefine stage for fine motion. In this case, the reticle coarse stagecorresponds to the translation stage table 100, with one degree offreedom. The reaction force generated by the motion of the reticle stagecan be mechanically released to the ground through a reticle stage frameand the isolator 54, in accordance with the structures described in JPHei 8-330224 and U.S. Pat. No. 5,874,820, the entire contents of both ofwhich are incorporated by reference herein. The light is focused by aprojection optical system (lens assembly) 46 supported on a projectionoptics frame and released to the ground through isolator 54. The lensassembly 46 may include transmitting glass elements (refractive),reflecting mirrors (reflective) or a combination of the two(catadioptric).

An interferometer 56 is supported on the projection optics frame anddetects the position of the wafer stage 51 and outputs the informationof the position of the wafer stage 51 to the system controller. A secondinterferometer 58 is supported on the projection optics frame anddetects the position of the reticle stage and outputs the information ofthe position to the system controller. The system controller controls adrive control unit to position the reticle R at a desired position andorientation relative to the wafer W or the projection optics 46.

There are a number of different types of photolithographic devices whichcan implement the present invention. For example, apparatus 40 maycomprise an exposure apparatus that can be used as a scanning typephotolithography system, which exposes the pattern from reticle R ontowafer W with reticle R and wafer W moving synchronously. In a scanningtype lithographic device, reticle R is moved perpendicular to an opticalaxis of projection optics 46 by reticle stage and wafer W is movedperpendicular to an optical axis of projection optics 46 by waferpositioning stage 52. Scanning of reticle R and wafer W occurs whilereticle R and wafer W are moving synchronously but in oppositedirections along mutually parallel axes parallel to the x-axis.

Alternatively, exposure apparatus 40 can be a step-and-repeat typephotolithography system that exposes reticle R while reticle R and waferW are stationary. In the step and repeat process, wafer W is in a fixedposition relative to reticle R and projection optics 46 during theexposure of an individual field. Subsequently, between consecutiveexposure steps, wafer W is consecutively moved by wafer positioningstage 52 perpendicular to the optical axis of projection optics 46 sothat the next field of semiconductor wafer W is brought into positionrelative to projection optics 46 and reticle R for exposure. Followingthis process, the images on reticle R are sequentially exposed onto thefields of wafer W so that the next field of semiconductor wafer W isbrought into position relative to projection optics 46 and reticle R.

However, the use of apparatus 40 provided herein is not limited to aphotolithography system for semiconductor manufacturing. Apparatus 40(e.g., an exposure apparatus), for example can be used as an LCDphotolithography system that exposes a liquid crystal display devicepattern onto a rectangular glass plate or a photolithography system formanufacturing a thin film magnetic head.

In the illumination system 42, the illumination source can be g-line(436 nm), i-line (365 nm), KrF excimer laser (248 nm), ArF excimer laser(193 nm), F₂ laser (157 nm) or EUV (13.5 nm).

With respect to projection optics 46, when far ultra-violet rays such asthe excimer laser is used, glass materials such as quartz and fluoritethat transmit far ultra-violet rays are preferably used. When the F₂type laser, projection optics 46 should preferably be eithercatadioptric or refractive (a reticle should also preferably be areflective type). When extreme ultra-violet (EUV) rays or x-rays areused the projection optics 46 should preferably be fully reflective, asshould the reticle.

Also, with an exposure device that employs vacuum ultra-violet radiation(VUV) of wavelength 200 nm or shorter, use of the catadioptric typeoptical system can be considered. Examples of the catadioptric type ofoptical system include the disclosure Japan Patent ApplicationDisclosure No. 8-171054 published in the Official Gazette for Laid-OpenPatent Applications and its counterpart U.S. Pat. No. 5,668,672, as wellas Japanese Patent Application Disclosure No. 10-20195 and itscounterpart U.S. Pat. No. 5,835,275. In these cases, the reflectingoptical device can be a catadioptric optical system incorporating a beamsplitter and concave mirror. Japanese Patent Application Disclosure No.8-334695 published in the Official Gazette for Laid-Open PatentApplications and its counterpart U.S. Pat. No. 5,689,377 as well asJapanese Patent Application Disclosure No. 10-3039 and its counterpartU.S. Pat. No. 5,892,117 also use a reflecting-refracting type of opticalsystem incorporating a concave mirror, etc., but without a beamsplitter, and can also be employed with this invention. The disclosuresin the above-mentioned U.S. patents, as well as the Japanese patentapplications published in the Office Gazette for Laid-Open PatentApplications are incorporated herein by reference in their entireties.

Further, in photolithography systems, when linear motors that differfrom the motors shown in the above embodiments (see U.S. Pat. Nos.5,623,853 or 5,528,118) are used in one of a wafer stage or a reticlestage, the linear motors can be either an air levitation type employingair bearings or a magnetic levitation type using Lorentz force orreactance force. Additionally, the stage could move along a guide, or itcould be a guideless type stage that uses no guide. The disclosures inU.S. Pat. Nos. 5,623,853 and 5,528,118 are incorporated herein byreference in their entireties.

Alternatively, one of the stages could be driven by a planar motor,which drives the stage by electromagnetic force generated by a magnetunit having two-dimensionally arranged magnets and an armature coil unithaving two-dimensionally arranged coils in facing positions. With thistype of driving system, either one of the magnet unit or the armaturecoil unit is connected to the stage, and the other unit is mounted onthe moving plane side of the stage.

Movement of the stages as described above generates reaction forces thatcan affect performance of the photolithography system. Reaction forcesgenerated by the wafer (substrate) stage motion can be mechanicallyreleased to the floor (ground) by use of a frame member as described inU.S. Pat. No. 5,528,118 and published Japanese Patent ApplicationDisclosure No. 8-166475. Additionally, reaction forces generated by thereticle (mask) stage motion can be mechanically released to the floor(ground) by use of a frame member as described in U.S. Pat. No.5,874,820 and published Japanese Patent Application Disclosure No.8-330224. The disclosures in U.S. Pat. Nos. 5,528,118 and 5,874,820 andJapanese Patent Application Disclosure No. 8-330224 are incorporatedherein by reference in their entireties.

As described above, a photolithography system according to the abovedescribed embodiments can be built by assembling various subsystems insuch a manner that prescribed mechanical accuracy, electrical accuracyand optical accuracy are maintained. In order to maintain the variousaccuracies, prior to and following assembly, every optical system isadjusted to achieve its optical accuracy. Similarly, every mechanicalsystem and every electrical system are adjusted to achieve theirrespective mechanical and electrical accuracies. The process ofassembling each subsystem into a photolithography system includesmechanical interfaces, electrical circuit wiring connections and airpressure plumbing connections between each subsystem. Needless to say,there is also a process where each subsystem is assembled prior toassembling a photolithography system from the various subsystems. Once aphotolithography system is assembled using the various subsystems, totaladjustment is performed to make sure that every accuracy is maintainedin the complete photolithography system. Additionally, it is desirableto manufacture an exposure system in a clean room where the temperatureand humidity are controlled.

Further, semiconductor devices can be fabricated using the abovedescribed systems, by the process shown generally in FIG. 28. In step2801 the device's function and performance characteristics are designed.Next, in step 2802, a mask (reticle) having a pattern is designedaccording to the previous designing step, and in a parallel step 2803, awafer is made from a silicon material. The mask pattern designed in step2802 is exposed onto the wafer from step 2803 in step 2804 by aphotolithography system described hereinabove consistent with theprinciples of the present invention. For example, the exposing the maskpattern onto the wafer may comprise one or more of the processes of theinvention described above with respect to FIGS. 11A-13. In step 2805,the semiconductor device is assembled (including the dicing process,bonding process and packaging process), then finally the device isinspected in step 2806.

FIG. 29 illustrates a detailed flowchart example of the above-mentionedstep 1304 in the case of fabricating semiconductor devices. In step 2911(oxidation step), the wafer surface is oxidized. In step 2912 (CVDstep), an insulation film is formed on the wafer surface. In step 2913(electrode formation step), electrodes are formed on the wafer by vapordeposition. In step 2914 (ion implantation step), ions are implanted inthe wafer. The above-mentioned steps 2911-2914 form the preprocessingsteps for wafers during wafer processing, and selection is made at eachstep according to processing requirements.

At each stage of wafer processing, when the above-mentionedpreprocessing steps have been completed, the following post-processingsteps are implemented. During post-processing, initially in step 2915(photoresist formation step), photoresist is applied to a wafer. Next,in step 2916 (exposure step), the above-mentioned exposure apparatus isused to transfer the circuit pattern of a mask (reticle) to a wafer.Step 2916, the exposure step, may comprise one or more of the processesof the invention described above with respect to FIGS. 11A-13. Then, instep 2917 (developing step), the exposed wafer is developed, and in step2918 (etching step), parts other than residual photoresist (exposedmaterial surface) are removed by etching. In step 2919 (photoresistremoval step), unnecessary photoresist remaining after etching isremoved. Multiple circuit patterns are formed by repetition of thesepre-processing and post-processing steps.

The foregoing examples have been provided for the purpose of explanationand are in no way to be construed as limiting of the present invention.While the present invention has been described with reference toexemplary embodiments, it is understood that the words, which have beenused herein, are words of description and illustration, rather thanwords of limitation. Changes may be made, within the purview of theappended claims, as presently stated and as amended, without departingfrom the scope and spirit of the present invention in its aspects.Although the present invention has been described herein with referenceto particular means, materials and embodiments, the present invention isnot intended to be limited to the particulars disclosed herein; rather,the present invention extends to all functionally equivalent structures,methods and uses, such as are within the scope of the appended claims.

What is claimed:
 1. A method, comprising exposing patterns on a wafer toradiation and predicting a critical dimension map of the patternsexposed on a wafer by a scanner which generates signals during theexposure of the patterns on the wafer, the critical dimension map beingpredicted from the signals generated by the scanner during exposure ofthe patterns on the wafer, wherein the critical dimension map ispredicted in situ with respect to the scanner, and further comprisingcreating a dose map and a defocus map, wherein the critical dimensionmap is predicted using the dose map and the defocus map.
 2. The methodof claim 1, wherein the signals are representative of exposure doseduring the exposure of the wafer.
 3. The method of claim 1, wherein thesignals are representative of defocus during the exposure of the wafer.4. The method of claim 1, wherein the signals are representative of lensthermal loads during the exposure of the wafer.
 5. The method of claim1, wherein the signals are representative of mechanical movement ofstages of the scanner during the exposure of the wafer.
 6. The method ofclaim 1, wherein the predicting the critical dimension map comprisesdefining at least one relationship between a critical dimension of thepattern exposed on the wafer and the scanner defocus during the exposureof the patterns on the wafer.
 7. The method of claim 1, whereinpredicting the critical dimension map comprises defining at least onerelationship between a critical dimension of the pattern exposed on thewafer and the scanner dose during the exposure of the patterns on thewafer.
 8. The method of claim 1, wherein predicting the criticaldimension map comprises defining at least one relationship between acritical dimension of the pattern exposed on the wafer and mechanicalmovement of stages of the scanner during the exposure of the patterns onthe wafer.
 9. The method of claim 1, wherein predicting the criticaldimension map comprises defining at least one relationship between acritical dimension of the pattern exposed on the wafer and the scannerlens thermal loads during the exposure of the patterns on the wafer. 10.The method of claim 1, wherein predicting the critical dimension mapcomprises creating a dose map from the signals generated by the scannerduring the exposure of the patterns on the wafer.
 11. The method ofclaim 1, wherein predicting the critical dimension map comprisescreating a defocus map from the signals generated by the scanner duringthe exposure of the patterns on the wafer.
 12. The method of claim 1,wherein predicting the critical dimension map comprises creating a lensthermal loads map from the signals generated by the scanner during theexposure of the patterns on the wafer.
 13. The method of claim 1,wherein predicting the critical dimension map comprises creating a mapof mechanical movement of stages of the scanner from signals generatedby the scanner during the exposure of the patterns on the wafer.
 14. Themethod of claim 1, wherein predicting the critical dimension map furthercomprises generating, in situ to the scanner, the critical dimension mapfrom a combination of: (i) a predefined relationship between a criticaldimension of the pattern exposed on the wafer and at least one of thefollowing characteristics of the scanner: defocus; dose; thermal loads;and of mechanical movement of stages of the scanner; and (ii) one ormore dose, defocus, thermal load and mechanical operation map or mapscreated from the signals generated by the scanner during the exposure ofthe patterns on the wafer.
 15. A system for predicting pattern criticaldimensions in a lithographic exposure process which exposes patterns ona wafer to radiation, comprising: a computing device configured topredict a critical dimension map of the patterns exposed on a wafer by ascanner which generates signals during the exposure of the patterns onthe wafer, the critical dimension map being predicted from the signalsgenerated by the scanner during the exposure of the patterns on thewafer, wherein the critical dimension map is predicted in situ withrespect to the scanner, and wherein the critical dimension map ispredicted using a dose map and a defocus map.
 16. The system of claim15, wherein the signals are representative of at least one of: exposuredose during the exposure of the wafer, and defocus during the exposureof the wafer.
 17. A computer program product comprising program codestored in a computer readable medium that, when executed on a computingdevice, in conjunction with a lithographic exposure process whichexposes patterns on a wafer to radiation, causes the computing deviceto: predict a critical dimension map of patterns exposed on a wafer by ascanner which generates signals during the exposure of the patterns onthe wafer, the critical dimension map being predicted from signalsgenerated by the scanner during exposure of the patterns on the wafer,wherein the critical dimension map is predicted using a dose map and adefocus map.
 18. The computer program product of claim 17, wherein: thesignals are representative of at least one of: the scanner exposure doseduring the exposure of the wafer, and the scanner defocus during theexposure of the wafer; and the critical dimension map is predicted insitu with respect to the scanner.